Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 1,305,800
Mortgage and asset-backed securities, amortized cost	[1]	6,171,500
Debt securities with maturity dates, amortized cost:
One year or less		557,100
Over one through five years		3,625,900
Over five through ten years		3,053,700
Over ten years		2,585,000
Debt securities, amortized cost		15,993,207	$ 15,727,143
Short-term investments due in one year or less, fair value		1,305,804	1,142,258
Mortgage and asset-backed securities, fair value	[1]	5,968,500
Debt securities with maturity dates, fair value:
One year or less		557,600
Over one through five years		3,556,200
Over five through ten years		2,938,800
Over ten years		2,537,500
Total debt securities, fair value		$ 15,558,624	$ 16,061,560

[1] Mortgage and asset-backed securities by their nature do not generally have single maturity dates.